A method of participating by a purchaser in a plurality of negotiations for purchasable items, where an item may be a good or a service, in order to obtain one of a plurality of alternative collections of complementary purchasable items, comprising maintaining a probabilistic model of said negotiations and making bids or offers to increase the probability of success in negotiations for purchasable items in one of said alternative collections determined from current states of said negotiations and said probabilistic model as providing a largest expectation of value to the purchaser.